Feature identification for metrological analysis

ABSTRACT

A method for selecting a set of features for monitoring a lithography process using a reticle, by identifying a set of candidate features, defining control regions around the candidate features, performing substrate level analysis using the reticle with different settings for the lithography process, determining which of the candidate features are most changed by the different settings, ranking the candidate features according to how much they are changed, and selecting the test set of features from those candidate features that are most changed.

FIELD

This application claims all rights to and priority on U.S. provisionalapplication Ser. No. 61/102,270 filed 2008 Oct. 2. This inventionrelates to the field of integrated circuits. More particularly, thisinvention relates to selecting appropriate sites on integrated circuitsfor metrological process evaluation.

BACKGROUND

The fabrication of integrated circuits relies heavily on lithographicprocesses. As the term is used herein, “integrated circuit” includesdevices such as those formed on monolithic semiconducting substrates,such as those formed of group IV materials like silicon or germanium, orgroup III-V compounds like gallium arsenide, or mixtures of suchmaterials. The term includes all types of devices formed, such as memoryand logic, and all designs of such devices, such as MOS and bipolar. Theterm also comprehends applications such as flat panel displays, solarcells, and charge coupled devices.

Lithography for current node sizes (45 nanometers and below) isparticularly challenging. The integrated circuit designs at this sizeexhibit high reticle error enhancement factors, and can print reliablyonly over relatively narrow focus-exposure process windows. To ensurethat the fabrication process is under control, a given number of themost challenging features within the design are selected for a detailedmetrology analysis. These features are carefully measured, such as witha scanning electron microscope, and the results are analyzed to ensurethat the substrates have been printed correctly. It is important tochoose the most error-sensitive (fragile) features in the design,because this metrology analysis is used as an early detection system forany problems in the lithography process.

One problem is that modern integrated circuits have so many fragilefeatures in their design that it is difficult to choose those that arethe most valuable for the metrology analysis. Optical proximitycompensation verification is currently used to determine the features onwhich to perform the metrology analysis. This is a method by which thedesign of the reticle, including the optical proximity compensationdecorations, is input into a lithographic simulation to determine whichfeatures are particularly fragile. Those fragile features are thenidentified as candidates for the substrate metrology analysis.

The optical proximity compensation verification process tends togenerate an extreme number of potential features, while providingrelatively little information as to how to determine which of thosepotential features are the most valuable. To overcome this problem, morefeatures than are really needed are selected for analysis, to try to geta reasonable statistical probability of selecting at least one of themore fragile features. This is time consuming and expensive. However,the most fragile features would not otherwise be selected out of themany thousands of identified features, so the earliest warning ofprocess deviations would not otherwise be achieved using this method.

What is needed, therefore, is a method that overcomes problems such asthose described above, at least in part.

SUMMARY

The above and other needs are met by a method for selecting a set offeatures for monitoring a lithography process using a reticle, byidentifying a set of candidate features, defining control regions aroundthe candidate features, performing substrate level analysis using thereticle with different settings for the lithography process, determiningwhich of the candidate features are most changed by the differentsettings, ranking the candidate features according to how much they arechanged, and selecting the test set of features from those candidatefeatures that are most changed.

Thus, the present methods provide the ability to properly prioritize thefeatures that are the most valuable for metrology analysis, using bothreticle design data and information from the actual reticle itself, andthereby provide the most accurate and earliest warning of substratelithography process deviations. These methods reduce the number offeatures that are selected for analysis, by recommending only the mostfragile features, instead of merely relying on (what is hoped to be) astatistically significant sampling of potential features. This in turnlowers the time and cost of the metrology analysis, because fewerfeatures are inspected. In addition, a better early warning system ofpotential lithography problems is provided, because the most fragilefeatures are actually selected for inspection.

In various embodiments according to this aspect of the invention, thestep of identifying the set of candidate features is performed usingoptical proximity compensation verification for design data for thereticle. Alternately, the step of identifying the set of candidatefeatures is performed by an inspection of the reticle. In someembodiments the different settings comprise a matrix of focus andexposure settings. In some embodiments the step of performing substratelevel analysis is accomplished by exposing test substrates with thereticle, and in other embodiments the step of performing substrate levelanalysis is accomplished using a processor and inspection data from thereticle. In some embodiments the step of determining which candidatefeatures are most changed includes determining which candidate featuresare changed by the smallest changes in the settings for the lithographyprocess. Some embodiments include the step of monitoring the lithographyprocess by performing metrology analysis on the test set of features onproduction substrates that are exposed with the reticle.

According to another aspect of the invention there is described a methodfor monitoring a lithography process, by exposing a set of testsubstrates with a reticle using different settings for the lithographyprocess, inspecting the exposed test substrates to detect which featuresare most changed by the different settings, selecting a test set offeatures from those features that are most changed, and monitoring thelithography process by performing metrology analysis on the test set offeatures on production substrates that are exposed with the reticle.

BRIEF DESCRIPTION OF THE DRAWING

Further advantages of the invention are apparent by reference to thedetailed description when considered in conjunction with the FIGURE,which depicts a flow chart of a method according to an embodiment of thepresent invention.

DETAILED DESCRIPTION

The inventors have determined that the prior art methods do not takeinto consideration the construction of the actual, physical reticle thatis used for printing the substrates (using the design information fromthe reticle only), and thus do not recommend the best features foranalysis. Methods according to various embodiments of the presentinvention select those features on the substrate that are the mostvaluable for metrology analysis based on both the design information forthe reticle and the details of the actual reticle that is used in theprinting of that substrate. Identifying substrate metrology analysispositions by using reticle construction errors in addition to thereticle design data has not been previously known.

In other words, a given feature in an integrated circuit design mightnot look fragile, but might be fragile as actually printed on thesubstrate, because of differences between the design of the reticle andthe actual physical condition of the reticle. These differences might bebrought about such as by reticle fabrication defects or other issues.The most fragile features, and thus the most valuable features formetrology analysis, are those features that are affected the most by thesmallest variances in the lithography process.

With reference now to the FIGURE, the first step of the method 10,according to one embodiment of the invention, is to identify thepotential features for analysis, as given in block 12. This can be doneeither in a separate step such as with an optical proximity compensationverification tool, or it can be accomplished during an inspection of theactual reticle. In either case, this step is performed by simulating thelithography operation across the focus-exposure process window (oracross the critical dimension window) for the fully optical proximitycompensation decorated reticle or design. The features that fail atfocus and exposure values closest to the nominal values are designatedas potential features for analysis. This step in one embodiment isperformed on a reticle inspection tool, which is a specializedprocessor-based tool capable of imaging a reticle and processing theimage and other data.

The second step of the method 10 is to generally locate control regionsthat surround the locations of these identified features, as given inblock 14. The third step is a full focus-exposure process windowinspection at the substrate plane for the control regions, as given inblock 16. Those features that are not properly formed are identified asbeing sensitive to the process within the process window, as given inblock 18. The sensitivity of these features is ranked according to howslight a variation from the nominal settings of the lithography processis required to produce the defect in the feature, as given in block 20.Position information for some number of the features in the prioritizedlist is output, and a desired number of the most sensitive (mostfragile) features are selected as the metrology analysis set, as givenin block 22.

In this manner, the total number of substrate metrology features isreduced from the prior art, because there is better knowledge about thesensitivity of each feature. The methods generally require an additionalamount of work up-front, but this is more than compensated for becauseit is only required once for each reticle-process combination, andgreatly reduces the amount of work that needs to be repetitivelyperformed later on during metrology analysis.

According to another embodiment of the invention, the first step ofidentifying a universe of potential features as described above is notperformed. Instead, the substrate plane inspection is performed first,across either the focus-exposure process window or across the criticaldimension window, and for the entire reticle. In one embodiment, ratherthan tracking all of the fragile features that are found, the featureidentification process keeps track of a running list of only a givennumber of the most fragile features as the process progresses. In thismanner, the identification process is performed at a very high level ofsensitivity without producing a list with an overwhelming number offragile features.

EXAMPLE 1

-   (A) Identify an initial list of features using optical proximity    compensation verification.-   (B) Define control regions around the features in the initial list.-   (C) Perform a substrate plane inspection analysis in the control    regions on the substrate using the actual reticle. The substrate    plane inspection analysis includes:    -   (i) Reticle pattern recovery using the transmitted and reflected        light images acquired from the inspection station,    -   (ii) A computational lithography translation of the recovered        reticle pattern down to the aerial image plane in resist, and    -   (iii) A resist/etch model to determine the actual impact on the        substrate.    -   This inspection can be performed across the full focus-exposure        process window to ensure that the full impact of any reticle        defect is incorporated.-   (D) The affects on the substrate of the features identified are    prioritized, and those with greater impact are used for metrology    analysis to monitor the lithography process. The prioritization    process could use process window width and location, maximum    critical dimension impact, specific reticle error enhancement    factors, or other factors.

Thus, the present methods provide the ability to properly prioritize thefeatures that are the most valuable for metrology analysis, and therebyprovide the earliest warning of substrate fabrication processdeviations. These methods reduce the number of features that areselected for analysis, by recommending only the most fragile features,instead of merely relying on (what is hoped to be) a statisticallysignificant sampling of potential features. This in turn lowers the timeand cost of the metrology analysis, because fewer features areinspected. In addition, a better early warning system of potentiallithography problems is provided, because the most fragile features areactually selected for inspection.

The foregoing description of preferred embodiments for this inventionhas been presented for purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed. Obvious modifications or variations are possible inlight of the above teachings. The embodiments are chosen and describedin an effort to provide the best illustrations of the principles of theinvention and its practical application, and to thereby enable one ofordinary skill in the art to utilize the invention in variousembodiments and with various modifications as are suited to theparticular use contemplated. All such modifications and variations arewithin the scope of the invention as determined by the appended claimswhen interpreted in accordance with the breadth to which they arefairly, legally, and equitably entitled.

1. A method for monitoring a lithography process, the method comprisingthe steps of: exposing a set of test substrates with a reticle usingdifferent settings for the lithography process, inspecting the exposedtest substrates to detect which features are most changed by thedifferent settings, selecting a test set of features from those featuresthat are most changed, and monitoring the lithography process byperforming metrology analysis on the test set of features on productionsubstrates that are exposed with the reticle.
 2. A method for selectinga set of features for monitoring a lithography process using a reticle,the method comprising the steps of: identifying a set of candidatefeatures, defining control regions around the candidate features,performing substrate level analysis using the reticle with differentsettings for the lithography process, determining which of the candidatefeatures are most changed by the different settings, ranking thecandidate features according to how much they are changed, and selectingthe test set of features from those candidate features that are mostchanged.
 3. The method of claim 2, wherein the step of identifying theset of candidate features is performed using optical proximitycompensation verification for design data for the reticle.
 4. The methodof claim 2, wherein the step of identifying the set of candidatefeatures is performed by an inspection of the reticle.
 5. The method ofclaim 2, wherein the different settings comprise a matrix of focus andexposure settings.
 6. The method of claim 2, wherein the step ofperforming substrate level analysis is accomplished by exposing testsubstrates with the reticle.
 7. The method of claim 2, wherein the stepof performing substrate level analysis is accomplished using a processorand inspection data from the reticle.
 8. The method of claim 2, whereinthe step of determining which candidate features are most changedcomprises determining which candidate features are changed by thesmallest changes in the settings for the lithography process.
 9. Themethod of claim 2, further comprising the step of monitoring thelithography process by performing metrology analysis on the test set offeatures on production substrates that are exposed with the reticle.